This invention relates to a method for starting a pump and a device therefor, and more particularly to the improvements in a method for starting a high water-head centrifugal pump, for example a pump-hydraulic turbine and a device therefor.
For example, a pump-hydraulic turbine, in general, includes essentially; a runner directly connected to a rotary main shaft coupled to a generator motor and a starting electric motor and adapted to rotate integrally with the shaft; a plurality of guide vanes surrounding the runner; a draft tube provided below the runner; a casing entirely encompassing the plurality of guide vanes; a pressurized-water iron tube coupled to the casing; a main valve provided midway in the pressurized-water iron tube; a by-pass valve for communicating the upstream and downstream sides of the pressurized-water iron tube with each other; an air compressor for feeding compressed air by way of a passage to the runner chamber which houses the runner therein; an air supply valve provided midway in said passage; a water-level controlling device opening and closing the air supply valve for adjusting a level of water in the runner chamber; a pressure sensitive switch adapted to be actuated upon sensing the pressure which prevails around the runner, to thereby actuate associated means; a drain valve for discharging from the runner chamber leaking water introduced from the plurality of guide vanes into the runner chamber during the idle running of the runner; a cooling water valve for feeding cooling water during the idling of the runner; a passage with a purge valve, through which is discharged air dwelling in the portion of the pressurized-water iron tube between the main valve and the runner; and a leaking-water supply passage with a leaking water supply valve, through which is supplied to the casing water which leaks into the runner chamber through gaps between guide vanes when completely closed.
In the pump-hydraulic turbine thus constructed, the below-described procedures have hitherto been taken for starting pumping operation thereof. To begin with, the main valve, by-pass valve and a plurality of guide vanes are maintained in a completely closed position, with the runner maintained stationary. Then, the air supply valve connected to the air compressor is brought into an open position, and at the same time, the drain valve connected to the runner chamber as well as the leaking-water supply valve connected to the pressurized-water iron tube leading to the casing are both brought into an open position. Since the air supply valve and the drain valve are brought into a full open position, compressed air is fed to the runner chamber, while water is discharged therefrom, so that, the level of water in the runner chamber is lowered. When the level of water in the runner chamber is lowered to a given level and the runner becomes exposed to compressed air, then the water level controlling device is actuated to close the air supply valve, whereby the feeding of compressed air to the runner chamber is interrupted. Thereafter, the water level controlling device is actuated to properly open and close the air supply valve for maintaining the level of water in the runner chamber constant. When the level of water in the runner chamber reaches a given level as a result of the substitution for compressed air, then the starting electric motor is energized, thereby starting rotation of the runner in the direction to effect the pump action. At this time, the runner is maintained exposed to compressed air, such that only the extremely small resistance is imposed on the runner when rotating. Furthermore, there is a possibility that pressurized water in the casing leaks through the gaps between guide vanes, which are completely closed, into the runner chamber, and in turn, compressed air in the runner chamber is admitted in the casing. The casing, however, may usually be maintained filled with water, because the leaking water supply valve is maintained open so as to feed to the casing water of an amount proportional to that of water leaking through gaps between guide vanes closed.
When the runner rotated by the runner starting electric motor reaches a specified r.p.m., the preparations for starting the water-pumping-up running is completed. Thus, the water-pumping-up starting instruction is given by runner-speed detecting means, whereupon the runner starting motor is deenergized, and in turn the generator motor is energized. At the same time, the water level controlling device is rendered inoperative, and the by-pass valve and the main valve are sequentially turned to an open position. As soon as the by-pass valve begins to open, the leaking water supply valve is closed. Subsequently, the drain valve is closed, while the exhaust valve is opened. Due to the exhaust valve being opened, water is fed from the draft tube to the runner chamber, whereby the runner chamber is filled with water, thereby raising the level of pressure of water in the vicinity of the runner periphery. When water pressure in the vicinity of the runner periphery is raised to a level allowing water-pumping-up action, then the pressure sensitive switch detects the pressure, thereby bringing the guide vanes into open position, whereby the water-pumping-up running is started.
The conventional method for starting pumping operation is satisfactory in a pump device whose water head is as low as 100m or thereabout, while with an increase in water head in the pump, a difficulty has been experienced with the conventional techniques for starting pumping operation.
Particularly, when a pump, whose water head is as high as 700 to 1000m, is widely used, as in the present time, water pressure in the pump is increased to two times or three times as high as that experienced in the past. In case of such a high water head pump, there is experienced in the intermediate stage providing for starting pumping operation i.e., in the condition where by-pass valve and main valve are opened, the guide vanes are closed and the drain valve is opened, pressure in the casing is considerably raised, with the resultant increase in the quantity of water leaking from the guide vanes closed. As a consequence, a great amount of water fills a part of the runner chamber, thus imposing extremely high resistance on the runner rotating.
As the countermeasure for solving the problem, it might be effective to increase the size of draining means leading to the runner chamber in proportion to the amount of water leaking from the guide vanes. However, to increase a diameter of a drain port leads to cavitation or vibrations during the normal water-pumping-up running of the pump, resulting in the lowered efficiency of the pump. Practically, increasing the number of drain tubes or the diameter of the drain tube, in most cases, is difficult or almost impossible from the standpoint of space.
Whether the amount of water leaking from the guide vanes is large or small is dependent on the precision of the guide vanes manufactured. In the initial stage of running of the pump, the amount of water leaking is only a little, but increases to a substantial degree during a long service of the pump due to severe mechanical looseness resulting from wear. To sustain the initial amount of water leaking, check and repair are required from time to time, resulting in a shortened running period of time. Thus, from the practical viewpoint, it is concluded impossible to prevent increase of the amount of leaking water.
If the amount of water leaking from the guide vanes increases, and a substantial amount of water stands in the runner chamber, then it follows that the water remains as pressurized water in the vicinity of the runner periphery, and part of the water makes ingress in the runner crown chamber and the runner band chamber which are defined above and below the runner. Pressurized water admitted in the runner band chamber is discharged therefrom, because the drain valve is maintained open at this stage, and consequently, pressure in the runner crown chamber becomes higher than that in the runner band chamber, whereas an extremely large thrust is exerted on the runner in the direction to urge the runner downwards. This inevitably and inadvantageously entails the use of a large, strong thrust bearing. Furthermore, water prevailing in the vicinity of the runner periphery is pressurized, and eventually water pressure, although the exhaust valve has been maintained open, is raised to a level allowing the water-pumping-up action before air is completely exhausted. Consequently, despite the air residing in the runner chamber, the pressure sensitive switch is actuated, to thereby bring the guide vanes to an open position, and thus, the water-pumping-up running is prematurely started. In the event that a great amount of air is contained in the water being pumped up, then, vibrations and noise of the pump result, as is well known, and there may arise a case where streams of water pumped up are blocked by air, with the failure to run the pump.
In short, if the conventional method for starting a pump is adopted for a high water head pump, such drawbacks are experienced that the severe running condition is imposed on the thrust bearing, as well as vibrations and noise of the pump arise.
These drawbacks are attributable to an increase in an amount of water leaking from the guide vanes into the runner chamber. For a smooth starting of the high water head pump, it is effective to reduce an amount of leaking water, or if reduction of the amount of leaking water is not allowed, to control pressure both in the runner crown chamber and in the runner band chamber, so as to prevent an extreme unbalance in pressures between both chambers, and to completely remove air from the runner chamber before pressurized water in the vicinity of the runner periphery is raised to a level as high as the water-pumping-up pressure.